Calibration and Reliability in Groundwater Modelling: Credibility of Modelling

(Proceedings of ModelCARE 2007 Conference, held in Denmark, September 2007). IAHS Publ. 320, 2008, 9-14.

 

Evaluation of Fickian and non-Fickian models for solute transport in porous media containing decimetre-scale preferential flow paths

 

MARCO BIANCHI1, CHUNMIAO ZHENG1, GEOFFREY R. TICK1
& STEVEN M. GORELICK2

1 Department of Geological Sciences, The University of Alabama, Tuscaloosa, Alabama 35487, USA

mbianchi@bama.ua.edu

2 Department of Geological & Environmental Sciences, Stanford University, Stanford, California 94305, USA

 

Abstract The effectiveness of the classical advection–dispersion model (ADM) to describe solute transport in heterogeneous aquifers has been challenged by many studies. In particular, it has been shown that the dual-domain single-rate mass transfer (DDSR) model is more appropriate than the ADM in characterizing solute transport in flow fields controlled by preferential flowpaths at the decimetre or smaller scales. In such situations the transport patterns, generally referred to as non-Fickian, are characterized by highly asymmetric plumes with early-time high concentration peaks and late-time low concentration tails. A recent develop­ment in non-Fickian transport is the continuous time random walk (CTRW) formulation. While this approach has been successfully applied to fit breakthrough curves from laboratory and field experiments, no systematic study has been conducted to test its applicability to simulate solute transport in porous media containing small-scale preferential flowpaths. In this study we conducted a detailed numerical experiment to evaluate the effectiveness of the ADM and two non-Fickian transport models (CTRW and DDSR) in repro­ducing the transport behaviour when small-scale preferential flowpaths are present in a binary heterogeneous system. Our reference is a 2-D synthetic aquifer characterized by a network of 10-cm wide high hydraulic conductivity channels, embedded in an otherwise homogeneous matrix. The contrast in hydraulic conduc­tivity between the channels and the remaining portion of the aquifer is 100:1. Numerical simulations were used to obtain accurate reference solutions for flow and contaminant transport in the channel-network system. Breakthrough curves and mass profiles from the reference solutions were compared with the results obtained from the ADM, DDSR and CTRW models to determine the most appropriate model for character­izing the transport behaviour in the synthetic aquifer controlled by decimetre-scale preferential flow paths.

 

Key words  advection–dispersion model; continuous time random walk; dual domain model; aquifer heterogeneity; preferential flowpaths